1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to correcting an inertial sensor.
2. Description of the Related Art
Generally, an inertial sensor denotes an acceleration sensor for measuring acceleration to calculate variation in the position of an object, and an angular velocity sensor, called a “gyroscope”, for measuring angular velocity to calculate variation in an angle of rotation of an object.
However, such an acceleration sensor is disadvantageous in that inherent error therein accumulates upon integration of measured acceleration with respect to time, thus causing a large difference between a calculated position and an actual position.
For example, the offset of the acceleration sensor is equal to the output of the acceleration sensor when no acceleration is applied, which should ideally be “0”. However, such an offset causes a slight error due to physical limitations in implementing the acceleration sensor, and may slightly change over time or with changing temperature. Such a slowly changing offset value of the acceleration sensor is called drift.
The drift of acceleration offset also exerts an influence when acceleration is applied, so that it is difficult to classify the output of the acceleration sensor into a part of the output caused by drift and a part of the output caused by applied acceleration.
FIGS. 1A to 1G are graphs showing motion trajectory when the above-described drift does not exist. FIG. 1A illustrates motion trajectory in which a rectangular path is formed counterclockwise in X-Y coordinates in a space.
FIGS. 1B and 1C illustrate graphs of position variation versus time with respect to x and y axes, respectively. FIGS. 1D and 1E illustrate graphs of velocity variation versus time with respect to x and y axes, respectively. FIGS. 1F and 1G illustrate graphs of acceleration variation versus time with respect to x and y axes, respectively.
FIGS. 2A to 2G are graphs showing motion trajectory when the above-described drift exists. FIG. 2A illustrates motion trajectory in which a rectangular path is formed clockwise in X-Y coordinates in a space.
FIGS. 2B and 2C illustrate graphs of position variation versus time with respect to x and y axes, respectively. FIGS. 2D and 2E illustrate graphs of velocity variation versus time with respect to x and y axes, respectively. FIGS. 2F and 2G illustrate graphs of acceleration variation versus time with respect to x and y axes, respectively. In this case, drift is assumed to be 0.01 m/sec2 
      (          ≅                        1          1000                ⁢        G              )    .
It can be seen that, when the graphs of FIGS. 1F and 1G are compared to the graphs of FIGS. 2F and 2G, a large difference does not exist therebetween. However, when the graphs of FIGS. 1D and 1E, which represent the graphs of FIGS. 1F and 1G integrated once, are compared to the graphs of FIGS. 2D and 2E, which represent the graphs of FIGS. 2F and 2G integrated once, a slight difference exists in an interval between one and two seconds with respect to the x axis and in an interval between two and three seconds with respect to the y axis. However, it can be seen that, when the graphs of FIGS. 1B and 1C, which represent the graphs of FIGS. 1D and 1E integrated once, are compared to the graphs of FIGS. 2B and 2C, which represent the graphs of FIGS. 2D and 2E integrated once, a greater difference exists. That is, it can be seen that, since drift exists, a large error is caused if integration is performed twice to obtain a position from acceleration.
FIG. 3A is a graph showing the comparison of other motion trajectories when drift exists and does not exist, which shows, for example, numeral “2” drawn in a space. There is a large difference between the case where drift exists and the case where drift does not exist. In FIGS. 3B, 3C and 3D, the motion trajectories shown in FIG. 3A are divided into components for x, y and z axes, respectively. The motion trajectories where drift exists are labeled “A” and the motion trajectories where drift does not exist are labeled “B”.
As described above, a large error is caused due to factors, such as drift, when motion trajectory is tracked using an inertial sensor.
Therefore, in order to minimize the error, variation in the position of an object is tracked by sensing that the velocity of the moving object is “0” using a predetermined method and correcting the integral value of acceleration to “0” whenever the velocity is “0”. This method is shown in FIG. 4A. For example, if the graph of FIG. 4A is assumed to be obtained by integrating acceleration measured by an inertial sensor, the integral value of acceleration at time T is corrected to “0” if the velocity of the moving object is sensed to be “0” at time T using a predetermined method.
As another method, there is a method disclosed in U.S. Pat. No. 6,292,751 (entitled “Positioning Refinement Algorithm”), wherein variation in the position of an object is tracked by sensing that the velocity of the moving object is “0” using a predetermined method, and subtracting a linear expression from the graph of velocity versus an entire time period so as to cause the integral value of acceleration to be “0” whenever velocity is “0”. The results of position variation tracking are shown in FIG. 4B. If the velocity of the object is sensed to be “0” at time T, a predetermined linear expression is subtracted from a velocity graph before correction with respect to all time intervals ranging from 0 to T, thus obtaining a velocity graph after correction.
The above methods work by correcting velocity or acceleration using information indicating that velocity is “0”. When velocity is “0”, motion does not occur in the direction of any axis. That is, the above methods are disadvantageous in that, since correction is performed only when velocity is “0” with respect to all axes, a large error may still accumulate.
Further, the above methods are problematic in that motion trajectory cannot be tracked for a period during which two or more characters are written. For example, when seven characters spelling “Leading” are written in cursive using the method disclosed in U.S. Pat. No. 6,292,751, motion trajectory is tracked in a form shown in FIG. 5. Although the time required for the writing is short, i.e., merely about 4.8 seconds, the characters spelling ‘Leading’ are not recognizable because of accumulated errors, as deduced from FIG. 5
Therefore, a correction method of minimizing error in an inertial sensor, without sensing the time at which the velocity of a moving object is “0” with respect to all axes, is required.